I live in Bluffton, SC and it get extremely hot in the attic during the summer (it hit 152 on my min/max thermometer last summer). Our air handlers are in the attic, so needless to say we get significant blasts of hot air coming out of the ductwork during the summer, and very cold air coming out in the winter. There is 10 inches of insulation on the attic floor but no insulation anywhere else (ductwork is flexible - r6). I have been looking at installing foam insulation to condition the attic so that I don't get the extreme temperatures coming through the ductwork. I have talked to a number of foam insulation installers in the area and can't seem to get a straight answer as to which way to go; open-cell or closed-cell. It gets extremely humid here during the summer. If we use closed-cell, where does the humidity in the attic already there go? Several of the contractors suggested using closed-cell because of the better r-rating. I also spoke to someone who pointed out that in the mornings when the roof tiles are moist from the nightly dew, vapor pressure from the sun on the moisture could push the moisture through the paper and roof into the attic. If we used closed-cell, where would the moisture go except against the roof. Could this cause the plywood to rot over time? If we used closed-cell, and sealed the ridge vent, how would the attic breathe? If we use open-cell is there a greater risk of mold and mildew during the summer months? I know it's a lot of questions, but I can't seem to get a straight answer from the contractors I have spoken to. Any help would be gratefully appreciated... Dave